This invention is in the field of plant molecular biology. More specifically, this invention pertains to nucleic acid fragments encoding sulfate assimilation proteins in plants and seeds.
Sulfate assimilation is the process by which environmental sulfur is fixed into organic sulfur for use in cellular metabolism. The two major end products of this process are the essential amino acids cysteine and methionine. These amino acids are limiting in food and feed; they cannot be synthesized by animals and thus must be acquired from plant sources. Increasing the level of these amino acids in feed products is thus of major economic value. Key to that process is increasing the level of organic sulfur available for cysteine and methionine biosynthesis.
Multiple enzymes are involved in sulfur assimilation. These include: High affinity sulfate transporter and low affinity sulfate transporter proteins which serve to transport sulfur from the outside environment across the cell membrane into the cell (Smith et al. (1995) PNAS 92(20):9373-9377). Once sulfur is in the cell sulfate adenylyltransferase (ATP sulfurylase) (Bolchia et al. (1999) Plant Mol. Biol. 39(3):527-537) catalyzes the first step in assimilation, converting the inorganic sulfur into an organic form, adenosine-5xe2x80x2 phosphosulfate (APS). Next, several enzymes further modify organic sulfur for use in the biosynthesis of cysteine and methionine. For example, adenylylsulfate kinase (APS kinase), catalyzes the conversion of APS to the biosynthetic intermediate PAPS (3xe2x80x2-phospho-adenosine-5xe2x80x2 phosphosulfate) (Arz et al. (1994) Biochim. Biophy. Acta 1218(3):447-452). APS reductase (5xe2x80x2 adenylyl phosphosulphate reductase) is utilized in an alternative pathway, resulting in an inorganic but cellularly bound (bound to a carrier), form of sulfur (sulfite) (Setya et al. (1996) PNAS 93(23):13383-13388). Sulfite reductase further reduces the sulfite, still attached to the carrier, to sulfide and serine O-acetyltransferase converts serine to O-acetylserine, which will serve as the backbone to which the sulfide will be transferred to from the carrier to form cysteine (Yonelcura-Sakakibara et al. (1998) J. Biolchem. 124(3):615-621 and Saito et al. (1995) J. Biol. Chem. 270(27):16321-16326).
As described, each of these enzymes is involved in sulfate assimilation and the pathway leading to cysteine biosynthesis, which in turn serves as an organic sulfur donor for multiple other pathways in the cell, including methionine biosynthesis. Together or singly these enzymes and the genes that encode them have utility in overcoming the sulfur limitations known to exist in crop plants. It may be possible to modulate the level of sulfur containing compounds in the cell, including the nutritionally critical amino acids cysteine and methionine. Specifically, their overexpression using tissue specific promoters will remove the enzyme in question as a possible limiting step, thus increasing the potential flux through the pathway to the essential amino acids. This will allow the engineering of plant tissues with increases levels of these amino acids, which now often must be added a supplements to animal feed.
The instant invention relates to isolated nucleic acid fragments encoding sulfate assimilation proteins. Specifically, this invention concerns an isolated nucleic acid fragment encoding a sulfite reductase and an isolated nucleic acid fragment that is substantially similar to an isolated nucleic acid fragment encoding a sulfite reductase. In addition, this invention relates to a nucleic acid fragment that is complementary to the nucleic acid fragment encoding sulfite reductase. An additional embodiment of the instant invention pertains to a polypeptide encoding all or a substantial portion of a sulfite reductase.
In another embodiment, the instant invention relates to a chimeric gene encoding a sulfite reductase, or to a chimeric gene that comprises a nucleic acid fragment that is complementary to a nucleic acid fragment encoding a sulfite reductase, operably linked to suitable regulatory sequences, wherein expression of the chimeric gene results in production of levels of the encoded protein in a transformed host cell that is altered (i.e., increased or decreased) from the level produced in an untransformed host cell.
In a further embodiment, the instant invention concerns a transformed host cell comprising in its genome a chimeric gene encoding a sulfite reductase, operably linked to suitable regulatory sequences. Expression of the chimeric gene results in production of altered levels of the encoded protein in the transformed host cell. The transformed host cell can be of eukaryotic or prokaryotic origin and include cells derived from higher plants and microorganisms. The invention also includes transformed plants that arise from transformed host cells of higher plants, and seeds derived from such transformed plants.
An additional embodiment of the instant invention concerns a method of altering the level of expression of a sulfite reductase in a transformed host cell comprising: a) transforming a host cell with a chimeric gene comprising a nucleic acid fragment encoding a sulfite reductase; and b) growing the transformed host cell under conditions that are suitable for expression of the chimeric gene wherein expression of the chimeric gene results in production of altered levels of sulfite reductase in the transformed host cell.
An addition embodiment of the instant invention concerns a method for obtaining a nucleic acid fragment encoding all or a substantial portion of an amino acid sequence encoding a sulfite reductase.